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Abstract

In this paper, athermal silicon waveguides using bridged subwavelength grating (BSWG) structures are proposed and investigated. The realization of temperature-independent BSWG waveguides for both polarizations is demonstrated numerically and experimentally. SU-8 polymer is used as the cladding material to compensate for the positive thermo-optic (TO) coefficient (dn/dT) of silicon. We investigate the dependence of the effective TO coefficient of BSWG waveguides on both the bridge width and grating duty cycle. The BSWG waveguides have a width of 490 nm, a height of 260 nm, and a grating pitch of 250 nm. Athermal behavior is achieved for both the transverse-magnetic (TM) and the transverse-electric (TE) polarized light for a variety of bridge width and duty cycle combinations. Furthermore, the BSWGs can be designed to be athermal for both TE and TM polarization simultaneously.

Figures (6)

a) Schematic of a subwavelength grating (SWG) waveguide core: W is the waveguide width, H is the waveguide height, a is the length of a silicon core segment, Λ is the grating pitch (period), k is the wavevector, and n1 and n2 are the core and cladding indices, respectively. b) Schematic of an athermal SWG waveguide.

(a) A schematic top view of the athermal BSWG waveguide. W1 is the width of the wider silicon core segments, W2 is the width of the bridging silicon segments, a is the length of the wider segments, and Λ is the grating pitch. The grating duty cycle (DC) is defined as a/Λ. (b, c) SEM images of fabricated BSWG waveguides. These waveguides were designed with W1 = 450 nm, H = 260 nm, and Λ = 250 nm: (b) with DC = 50% and W2 = 140 nm; (c) with DC = 65% and W2 = 80 nm.

The thermo-optic coefficient dependence on the wavelength. The TO coefficient of athermal BSWG waveguides are compared with the PW waveguide. The BSWG waveguides were designed nominally athermal at the wavelength λ = 1550 nm.